Medicare accountable care organizations (ACOs) that have participating provider organizations using a variety of electronic health record systems (EHRs) face care coordination challenges due to incomplete data sharing. Because their EHRs may be incompatible, ACO participants often rely on telephone calls and faxes to share data between provider organizations. Some ACOs have access to robust health information exchanges (HIEs), which provide access to consumer data, even when consumers received care from provider organizations outside the ACOs' networks. However, most ACOs had access to HIEs with partial or incomplete data, making it difficult to coordinate care for consumers who used a . . .